Nuclear magnetic resonance (NMR) is a physical phenomenon involving quantum mechanical magnetic properties of atomic nuclei in the presence of an applied, external magnetic field. NMR phenomena can be observed with an NMR spectrometer and used to study molecular physics, crystalline and non-crystalline materials. In particular, nuclear spin phenomena can be used to generate a spectrum comprised of a pattern of lines representing the various nuclear spins and spin interactions.
In order to perform an NMR experiment, a sample is placed in the external or B0 magnetic field to create a net magnetization in the sample. A radio-frequency (RF) field or B1 field is then applied to the sample to rotate the net magnetization in a pulse sequence. Sample coils that surround the sample not only create the B1 field for the pulse sequence, but also detect the NMR signal from the sample.
Single or multiple sample coil combinations can be used. The set of coils must be configured so that, for each nucleus to be observed, a resonance frequency similar to the Larmor frequency of the nucleus is created. Single coils may formed exclusively from wire (a mainly inductive element, which can be used, for example, for broad banded applications) or as a combination of inductive and capacitive elements that form a resonator at a given frequency. Since the presence of the sample affects the resonant frequency of the coils, the resonances have to be tuned for the specific sample being studied in order to achieve the highest signal-to-noise ratios. Another requirement of a sample coil is that the B1 field produced by the coil must be homogeneous over the volume of the sample. If the B1 field is not constant, the magnetization will be rotated by a distribution of rotation angles and the resulting spectra will be distorted.
NMR experiments can be performed on both liquid and solid samples. Spatial proximity and/or a chemical bond between two atoms can give rise to interactions between the nuclei of the atoms. In general, these interactions are orientation dependent. In an NMR experiment involving a liquid sample, Brownian motion of the molecules and atoms causes an averaging of anisotropic interactions. In such cases, these interactions can be neglected on the time-scale of the NMR experiment. However, in solid samples, for example crystals, powders and molecular aggregates, the anisotropic interactions between nuclei have a substantial influence on the behavior of a system of nuclear spins. In particular, in solid materials, the great number of interactions produces very broad and featureless NMR result lines. However, the interactions are time-dependent and can be averaged by physically spinning the sample (at high rotation speeds up to 80 kHz) at an inclination of the so-called magic angle (54.74°) with respect to the direction of the external B0 magnetic field. The averaging causes the normally broad lines become narrower, increasing the resolution for better identification and analysis of the spectrum.
To perform a magic angle spinning (MAS) nuclear magnetic resonance experiment, a sample is typically packed into a rotor that fits inside the sample coil and is rotated at high speed by an air turbine. The rotor is held in place by air bearings. The entire structure is then inserted into the bore of a high strength magnet. This design places stringent considerations on the sample coil size and location.
Due to the very restricted space between the air bearings and the high strength B1 fields and thus high power requirements, a number of coil designs are used to provide “optimal” performance. With the “best” filling factor in this configuration, a solenoid coil was the coil of choice for some time. In the last decade experiments on biosol id samples have been performed with the drawback of lossy (usually salty) samples that absorb energy and heat the sample while destroying the biomass inside. Several different coils have been developed including a “cross coil” version with some success. These two coil systems consists of two separate coils, one high frequency resonator with a reduced E-field (the E-field causes heating) and one highly efficient solenoid coil for the lower frequencies.
To make matters more complicated, many present day experiments require NMR probes with sample coils tuned to several different frequencies so that B1 energy at these frequencies can be applied simultaneously to the sample or at least applied sequentially without removing the sample from the magnet bore. For example, a typical triple resonant probe might have sample coils tuned to the Larmor frequencies of 13C, 15N and 1H atoms. At a B0 field strength of 18.8 Tesla, these Larmor frequencies correspond to 200, 80, and 800 MHz, respectively. Due to the large difference in Larmor resonant frequency between the 15N and 1H atoms, a two coil “cross coil” structure is generally used to separate the frequencies. Isolation of the three NMR signals generated during the NMR experiment is achieved using different approaches, including rejection traps, geometrically decoupled coils or transmission lines that pass different wavelengths.
However, the conventional two coil approach has significant problems with uniformity of the B1 field inside of the sample coil. More specifically, the high frequency and low frequency coils are not connected together so that a potential difference develops between the ends of the solenoid coil and the Helmholtz coils which can cause arcing. Experiments on solids need high B1 fields for long time intervals which also increases the chance of arcing in these applications. Therefore, in order to reduce arcing between the coils either the B1 field strength must be limited and/or a significant space must be left between the ends of solenoid coil and the high frequency resonator. Since the overall size of the coil structure is limited by other factors, the result is that the length of the solenoid coil is reduced. FIG. 1 is a graph of the B1 field strength inside of a conventional sample coil configuration. The horizontal axis is the position inside of the coils measured from one in millimeters with the center of the coils occurring at 6.8 mm. The vertical axis indicates the B1 field strength normalized to the field strength at the center of the coils. The graph represented by the filled diamonds is the B1 field strength at the 1H frequency; the graph represented by the hollow squares is the B1 field strength at the 13C frequency and the graph represented by the hollow triangles is the B1 field strength at the 15N frequency. As can be seen from the graphs, the field strength at the 1H frequency is relatively flat over the range of 3.3 mm to 10.3 mm. However the 13C and 15N field strengths fall off rapidly away from the center position of the coil system due to the restricted length of the coil. In general, a variation of the B1 field strength of more than ten percent is not tolerable for the reasons discussed above. Therefore, as shown in the figure, the usable area of the coil system extends only from 5.8 mm to 7.8 mm or a total of 2 mm. This usable area severely restricts the sample size.
Therefore, there is a need for an improved multiple resonant coil design.